The calcium-sensitive bioluminescent protein aequorin will be developed further as a tool for the detection and measurement of changes in intracellular calcium ion concentration in muscle and other excitable cells. Work will be directed toward three general goals: 1) Improvement of techniques for the purification and handling of aequorin in a form suitable for microinjection into single cells. 2) Better definition of the kinetics of reaction of aequorin with calcium. 3) Study of calcium transients in skeletal muscle, cardiac muscle, and other cells with particular attention to clarifying the mode of action inotropic drugs.